Eukaryotic protein translation initiation factor 4D (eIF-4D) contains one residue of hypusine and appears to be the only cellular protein with this one unique amino acid. Hypusine is produced posttranslationally by transfer of the butylamine portion of the polyamine spermidine to a lysine residue in the eIF-4D precursor and subsequent hydroxylation. These findings reveal a novel cellular metabolic pathway. Comparison of activity of mature eIF-4D and eIF-4D precursors that contain unmodified lysine in place of hypusine in methionyl-puromycin synthesis indicate that hypusine is essential for the activity of eIF-4D in this model protein synthesis initiation system. Studies are underway to relate the structure of hypusine to the physiological function of eIF-4D and to its mode of action in eukaryotic protein synthesis.